Subcellular life in the fast lane.

The video images resemble toy cars speeding along disconnected stretches of highway, where invisible children arbitrarily remove cars and return them to the track. With such videos of microscopic structures, scientists are working out the rules-of-the-road for transportation within a cell, as well as identifying the subcellular fuel, motors and roadways.

Nerve cells, with their long, thin axons and dendrites, present the greatest transport challenge. For more than a decade, biologists have recognized two transport processes, but their mechanisms have been a mystery. The movement of the axon's entire contents (the axoplasm) within the membrane is characterized as slow transport. Within that slowly flowing cytoplasm, some structures zip along, in either direction, at more than 100 times the slow-transport rate.

The complicated activity and the congestion within an axon make analysis difficult. A simplified experimental situation was recently developed at the Marine Biological Laboratory in Woods Hole, Mass., by Thomas S. Reese of the National Institutes of Health. He and colleagues now report that the faster transport uses adenosine triphosphate (ATP) as fuel, a complex of proteins as a motor, and microtubules, single filaments of the protein tubulin, as the roadways. The fast-transport system is distinct from the two types of cellular machinery previously described that produce intracellular movement in plants and animals, Reese says.

In their experimental system Reese and colleagues squeeze the axoplasm from a squid giant axon, which is about 1,000 times wider than any axon of a vertebrate. If ATP is present, filaments move away from the bulk of the material and adhere to a glass plate. The scientists use video-enhanced microscopy (SN:4/11/81, p.234) to view the filaments, which have diameters of 25 nanometers. At a Society for Neuroscience seminar last week in Washington, D.C., Reese showed video images of subcellular structures -- vesicles and mitochondria -- traveling along the scattered filaments.

From the video images, the scientists conclude that each vesicle and mitochondrion must have more than one attachment site, because they can switch from one filament to another nearby. In addition, each filament has more than one traffic lane, because organelles going in opposite directions can pass without colliding. Using antibodies, Reese and colleagues report in the February Cell that each filament is a single microtubule.

By isolating proteins from axoplasm, they have determined those essential to movement. "We now have a complex of proteins that seems to be the fast-transport motor," Reese says.